1. Field of the Invention
The present invention relates to a cylindrical molded article and an injection mold, and more particularly relates to a cylindrical molded article used in the lens barrel of a camera and an injection mold.
2. Description of the Related Art
One conventional type of lens barrel is a multistage retractable lens barrel. This kind of lens barrel is made up of a plurality of cylindrical members of different diameter. To convert the rotational motion of the cylindrical members into axial linear motion, three cam grooves and three cam pins that engage with these cam grooves are provided to a plurality of cylindrical members, for example. The cam grooves are usually formed on the inner peripheral part of the cylindrical members.
It is generally difficult to machine cam grooves on the inner peripheral part of cylindrical members. Therefore, the cylindrical members that make up a lens barrel are formed by injection molding. An injection molding apparatus mainly includes an injection mold, and an injection apparatus for injecting the molten molding material into the mold. The injection apparatus can be adjusted for molding material injection pressure and injection speed.
The injection mold is provided with a cavity, a sprue, a plurality of runners, and a plurality of gates. The cavity is a hollow space used for form a molded article. The sprue is a channel through which flows the molding material injected from the injection apparatus. The runners guide the molding material from the sprue to the cavity. The gates are constrictions for preventing the back-flow of the molding material from the cavity to the runners, and are disposed between the runners and the cavity. In the case of a cylindrical molded article, a plurality of runners are disposed at a constant pitch in the circumferential direction so that the molding material will flow evenly. A plurality of gates are also disposed at a constant pitch in the circumferential direction.
Injection molding mainly includes a temperature adjustment step in which the temperature of the metal mold is adjusted, a filling step in which the mold is filled with the molding material, and a pressure-holding cooling step in which the molded article is cooled inside the mold. In the pressure-holding cooling step, the pressure is held at a specific level by the injection apparatus. This causes molding material to be supplied to portions where heat shrinkage has occurred, and minimizes deformation of the molded article due to heat shrinkage.
However, the wall thickness of the cylindrical molded article is uneven because of the cam grooves. Consequently, the thicker and thinner parts cool at different rates in the pressure-holding cooling step, so the heat shrinkage varies from place to place. As a result, there is a decrease in the circularity of the cylindrical molded article, and the cylindrical molded article cannot be obtained as designed. Even though the pressure is maintained in the pressure-holding cooling step, it may be impossible to suppress deformation of the molded article, depending on the shape of the cam grooves.
If the cylindrical molded articles that make up a lens barrel have decreased circularity, the cam grooves will be offset in the radial direction from the designed positioned. As a result, the lens group supported by the plurality of cylindrical molded articles becomes out of position, and this adversely affects the optical performance of the imaging optical system. Also, if the cam grooves become misaligned in the radial direction with respect to the designed position, there will be greater sliding resistance between the cam grooves and the cam pins, which hinders smooth zoom operation. As a result, greater drive force is necessary, and this increases power consumption.
In view of this, as disclosed in Japanese Patents 3,523,249 and 2,995,509, correction of the injection mold is generally carried out in conventional injection molding. More specifically, with a conventional metal mold design, deformation due to heat shrinkage is predicted on the basis of experimentation or simulation. The mold is produced in the shape of the molded article according to the predicted amount of deformation. Next, a prototype is formed using the mold thus produced. The dimensions of the various parts of the prototype are measured, and the differences between the design and measured values are calculated. Metal mold correction is performed using this dimensional error as an offset value. In the case of cylindrical molded articles used in a lens barrel, the cavity of the mold is formed as a cylindrical hollow space that is not a true circle.
If there is a large difference between the design and measured values, then more of the mold has to be machined, so mold correction takes more time. Also, if there is a large difference between the design and measured values, then there is greater dimensional change in the various parts of a prototype formed with the corrected mold than with a prototype formed with the initial mold. Consequently, it is unlikely that a molded article will be obtained with the design values after just one mold correction.
Conversely, if there is a small difference between the design and measured values in a prototype produced by the initial mold, mold machining takes less time and it is more likely that a cylindrical molded article will be obtained with the design values after a single mold correction.
As discussed above, it is preferable with an injection mold for the error from the design values of a molded article to be kept as small as possible.
On the other hand, there is a need in the field of digital cameras for the main body to be as compact as possible to make the product more portable. More specifically, there is a need to reduce the size of the lens barrel, which is considered to be a major factor in obtaining a smaller overall size. One way to make a lens barrel smaller is to increase the change ratio of the focal distance in zooming. As this is done, the shape of the cam grooves becomes more complicated, and the difference in the wall thickness of the cylindrical molded articles increases. Consequently, reducing the size of a lens barrel leads to a decrease in dimensional precision in cylindrical molded articles.
However, with the prior art discussed above, all that was proposed was a method for measuring dimensional error, or a method for predicting dimensional error by simulation and factoring this error into the metal mold design.
When a cylindrical molded article having a cam groove is injection molded, there seems to be some kind of relationship between the circularity and shape of the cam groove, but the details of this relationship are not yet clear. Therefore, with a conventional injection mold, even if changes are made to the shape of the cam grooves, there is the risk that more mold corrections will be necessary.